Avian sarcoma viruses (ASV) provide a model system for the study of the mechanisms of DNA integration and the control of transcription of viral genes. A large amount of information is available regarding the genetic and physical structure of the ASV genome, the steps by which viral RNA is converted to a DNA intermediate, and the transcription of integrated viral DNA sequences. Although the integration of viral DNA into the host chromosome is an efficient event, very little information is available regarding the actual enzymatic functions required for covalent insertion of viral DNA. Our studies seek to define the DNA sequences, enzymes, and factors required for viral DNA integration. We have molecularly cloned the complete ASV DNA genome and are using such cloned DNA as reagents to examine the steps in viral DNA integration. We are propose to determine whether avian sarcoma viruses encode proteins that may be responsible, in part, for the site-specific integration of viral DNA. We will investigate the effect of site-directed mutations (within the long terminal repeats of the viral DNA) on the integration of viral DNA sequences. Finally, we will attempt to construct an in vitro assay for the integration of viral DNA.